There is a need to develop new methods for (1) the economic and environmentally acceptable recovery of uranium, plutonium, and certain other elements from radioactive, complex waste mixtures and (2) the conversion of the residual wastes to borosilicate waste glass. There is an economic incentive for recovery of expensive materials for reuse. The wastes must be converted into an environmentally acceptable waste form. Borosilicate glass is the preferred radioactive waste form worldwide. The removal of plutonium, enriched uranium, and high-enriched uranium (HEU) from some of these wastes may be required to make the waste acceptable for disposal. Because plutonium, enriched uranium, and HEU can cause safeguard problems and nuclear criticality problems for disposal sites, they may not be acceptable in high concentrations in final waste forms. Examples of materials containing plutonium and uranium include:
a. Uranium Fissile Wastes
Gaseous diffusion enrichment plants and certain fuel fabrication facilities have significant quantities of wastes with substantial quantities of enriched uranium. Criticality and safeguards issues may prevent disposal as low-level radioactive waste. Separation of the enriched uranium would create a saleable product and a low-level waste that can be disposed of.
b. Miscellaneous Spent Nuclear Fuels (SNFs)
Many of these SNFs are probably not acceptable for repository disposal because (1) over long periods of time the HEU may cause nuclear criticality in a geological repository, and (2) the chemical forms of the SNF are not suitable for long-term disposal. This SNF must be processed into an acceptable waste form. Most of these SNFs contain HEU that, if separated from the SNF, can be blended down with low-enriched uranium to produce valuable fuel for nuclear power reactors. Other materials included in this category are miscellaneous hot-cell wastes. Many of these wastes are from destructive analysis of SNF assemblies. The chemical variability of hot-cell wastes is much greater than for miscellaneous SNFs.
c. Plutonium Scrap and Residue
These materials are highly toxic. There are safeguard issues associated with the storage of such materials. Moreover, some of the materials are in chemically unstable forms that are unlikely to be acceptable for disposal. Plutonium is in excess supply; but if it can be recovered, the residuals minus the plutonium can be disposed of as transuranic waste, and the plutonium can then be put into special forms for long-term storage or disposal.
Many methods for recovering plutonium and uranium have been developed but these methods have difficulties in processing complex waste streams. Historically, it has often been uneconomical to recover these elements just for their value. In most cases in the past, these wastes have been placed in storage to be treated at some later time. However, this is no longer an acceptable policy.
The traditional approach to recovering plutonium and uranium from feed streams is to (1) dissolve the material in acid; (2) recover the desired elements from the acid by solvent extraction, ion exchange, or precipitation; and (3) convert the waste stream to an acceptable waste form--usually borosilicate glass. In recent years, nitric acid has been the preferred dissolution acid because it can be destroyed after its use, thus yielding gaseous nitrogen and oxygen while producing no additional waste. The best known of the separation processes is the PUREX process. The Plutonium Uranium Extraction Process (PUREX) is used for recovery of uranium and plutonium from various feed stocks. In the process, the feed is dissolved in nitric acid. The plutonium in the acid is in the +4 valence state while the uranium is in the +6 valence state as the UO.sub.2.sup.+2 ion. The aqueous acid stream is contacted with an organic stream with an organic solvent (such as kerosine) containing tributylphosphate. The organic does not dissolve into the aqueous stream and the aqueous stream does not dissolve into the organic stream. The uranium and plutonium are selectively extracted from the aqueous stream to the organic stream.
The organic stream is then contacted with a second nitric acid stream containing a reducing agent such as ferrous ion or hydroxylamine. The plutonium in the +4 valence state is converted to plutonium in the +3 valence state and extracts from the organic stream into the second aqueous stream. This second aqueous stream is relatively pure plutonium nitrate in an acid stream. Last, the organic stream is contacted with water. The uranium is extracted from the organic to the water producing a product uranium nitrate dissolved in water. The organic is recycled back to the beginning of the process to remove more plutonium and uranium from the acid feed. The base technology is efficient and economical when processing clean, oxide-like materials that dissolve quickly in nitric acid. This technology is used worldwide on an industrial scale. Unfortunately, there are major limitations with the technology which make it unsuitable in its current configurations for processing many waste streams. Some of the disadvantages are:
a. Many metals and high-fired ceramics cannot be dissolved by nitric acid at acceptable rates. Dissolution of waste materials can usually (but not always) be achieved by the total dissolution of such materials in a mixture of hydrofluoric and nitric acids followed by recovery of the desired elements from solution. This process adds fluorides into the waste stream which, in turn, create major additional disposal problems including major increases in waste volumes and poor-quality waste forms. For highly radioactive wastes, it is the extra costs to handle the higher waste volumes that often make this option prohibitively expensive. PA1 b. The presence of organics, other carbon-containing materials, or halides in feeds complicates the recovery of the desired elements and disposal of the waste acid streams after extraction of the product. The acid waste streams cannot be easily converted to an acceptable waste form (e.g., glass) if they contain organics, metals, or halides. If the waste acid stream contains such materials, these materials must often be removed before conversion of the acid waste to glass. Glasses are made from oxide or oxide like materials. PA1 providing a bath of molten boron oxide, B.sub.2 O.sub.3, and lead oxide, PbO, wherein a molten dissolution glass comprising xPbO:B.sub.2 O.sub.3 is formed; PA1 adding nuclear waste feed material to the molten dissolution glass to form a molten dissolution glass/waste mixture; wherein metals (except noble metals) which were in the waste are oxidized and the resultant metal oxides are dissolved into the molten mixture, molten lead is formed and separates from the glass/waste mixture, noble metals dissolve into the molten lead, the lead sinks to the bottom of the melter, halogen-containing compounds which were in the waste are converted to gaseous lead halides, and carbon-containing compounds (e.g. organic material) are oxidized to form carbon oxides and water; PA1 separating the gaseous halides from the molten mixture and contacting the gases with an aqueous scrubber solution of an alkali metal hydroxide to yield a soluble alkali metal halide and a lead-containing precipitate; PA1 returning the lead-containing precipitate from the scrubber to the molten glass/waste mixture; PA1 separating the molten lead, which contains dissolved noble metals, from the glass/waste mixture, and recovering the noble metals from the molten lead; PA1 adding carbon to the molten glass/waste mixture to remove lead oxide by converting it to lead and carbon oxides, wherein a boron oxide fusion melt is formed, essentially devoid of halides and organic material or other carbon containing material; PA1 dissolving the boron oxide fusion melt, which may contain U, Pu, and rare earth metals, in nitric acid; and PA1 separating and recovering U, Pu and rare earth metals from the acid solution.
A second class of wastes contain valuable elements worth recovery--rare earths. "Rare earths", as used herein, is intended to encompass the elements in the periodic table having atomic numbers 57-72, and will also be referred to as rare earth metals. Rare earths are used in electronic display screens and other applications. Currently, no good, economically viable recovery options exist for recovering these materials from manufactured products. Rare earths found in natural ore deposits are not extremely expensive. However, separating specific rare earths from complex rare earth mixtures is expensive. As a consequence, there is an incentive to recover rare earths from specific industrial waste streams or other waste material where only one or two rare earths are in the wastes.
Accordingly it is an object of the present invention to provide an improved method of converting complex nuclear waste into a form from which fissile materials may be easily recovered.
Another object of the invention is to provide a method by which rare earth metals can be easily recovered from complex nuclear waste materials, industrial waste or any other waste material.
A further object of the invention is to provide a method by which uranium (U) and plutonium (Pu) can be separated and recovered from complex nuclear waste materials.
It is another object of the invention to convert complex nuclear waste mixtures, industrial waste or other waste material into a borate fusion melt which is easily dissolvable in nitric acid, and from which U, Pu, and rare earth metals can be easily recovered.
It is yet another object of the invention to easily and economically recover U, Pu, and rare earth elements from complex nuclear waste or other waste materials, and to convert the waste remaining to a waste glass suitable for storage or disposal.